The invention is generally directed to electric motors in which fluctuating magnetic fields are used to produce force, work, torque or the like, and which include a magnetic flux return path. More particularly, the invention is directed to a novel electric motor design which allows more efficient operation and which includes a magnetic flux return path with very low loss and an extremely flexible design.
Electric motors are generally constructed with a rotor having permanent magnets thereon which produce a rotating magnetic field as the permanent magnets are rotated. The rotor may include a fixed magnetic flux return path, and is rotationally supported within a stator assembly. The stator assembly generally will comprise field windings which are energized to produce rotation of the rotor by means of the permanent magnets situated thereon. A magnetic flux return path situated about the field windings of the stator assembly completes a magnetic circuit for the flow of magnetic flux generated. In such a motor construction, the fluctuating magnetic fields produced by rotational movement of the permanent magnets associated with the rotor may lead to losses associated with eddy currents and hysteresis. For example, if the magnetic flux return path of the motor is such that magnetic saturation occurs, hysteresis losses may also be induced. In such motors, where fluctuating magnetic fields are utilized to produce torque and power in an output shaft, the magnetic flux is directed from one magnetic pole to the other magnetic pole of a permanent magnet found on a rotor through the flux return paths. As the magnetic field within the flux return path varies due to rotation of the permanent magnets, losses may be induced.
In motors using high frequency alternating currents, both eddy current losses and hysteresis losses become greater due to the greater flux density within the magnetic material of the flux return path. Additionally, with high frequency applications, skin effects and magnetic saturation become of greater concern. Both eddy current and hysteresis losses generate heat which presents a major design problem for the motor construction. To minimize hysteresis losses, previous applications have resorted to increasing the thickness of the flux return path to avoid magnetic saturation of the material.
Previous flux return path designs have also attempted to minimize eddy current losses by constructing the magnetic flux return of laminations or a number of thin iron plates which are insulated from one another. At low frequencies, eddy current losses are a function of the thickness of the laminations making up the core and flux return path of the motor. Similarly, at high frequencies, both eddy current losses and hysteresis losses may be dependent upon the thickness of the laminations due to skin penetration effects. Thus, in the design of the magnetic circuit in such motors, losses are generally minimized by providing a thick flux return path thereby avoiding saturation of the iron material normally used in their construction. The flux return path is made of a number of thin laminates from a material having a high magnetic permeability wherein very thin laminates are normally desirable. Unfortunately, the degree of thinness obtainable for the laminations is limited by construction and fabrication problems as well as by cost.
As an example, a slotless, brushless DC motor flux return path or backiron may be constructed of a plurality of laminations, designed to have a radial thickness of about 0.060 inches to accommodate a flux density which is limited to about 1.0 T. The flux density can be limited to 1.0 T when the rotor magnets have a flux density of about 0.4 T which is a typical value for high flux ceramic magnets. A backiron construction having the desired thickness may be constructed of multiple laminations having conventional thicknesses of about 0.014 to 0.025 inches. Thinner laminations are possible but add to the cost of fabrication. For a typical DC motor with an output of 50 watts at 20,000 RPM the backiron parameters conventionally employed are a stack length of 1.5 inches and a diameter of 2 inches. To form a radial thickness of about 0.060 inches, the backiron weight will be approximately 0.167 pounds which must be considered in the design of the motor. With a slotless brushless motor as described, the loss in the magnetic flux return path or backiron can be calculated as follows: EQU L.sub.BI =afB.sub.o.sup.2 +b(tfB.sub.o).sup.2 ( 2)
where L.sub.BI is the loss in the backiron in watts/kg, a and b are constants depending on the properties of the material, f equals the flux frequency in Hz., B.sub.0 equals the flux density in the backiron in Tesla, and t equals lamination thickness in millimeters. In the equation for calculating the loss in the backiron as described above, the values of a and b will be known. For example, using an AISI grade M-15 iron laminate, which is typically used for such laminations, these constants will have values of a=0.019 and b=6.2.times.10.sup.-4. Flux saturation levels in most iron materials occurs at approximately 1.2 to 1.6 T and therefore saturation effects will not be considered in this example as the flux density has been limited to about 1.0 T. For a typical DC motor construction with the backiron as described above and an output of 50 watts at 20,000 RPM, the backiron stack length of 1.5 inches would require about 107 laminations which are about 0.014 inches in thickness or approximately 60 laminations if the thickness is about 0.025 inches. Cost considerations would require choosing between better performance characteristics for thinner laminations or less cost for the thicker laminations. The backiron loss, L.sub.BI, for a construction using the lamination thickness of 0.025 inches with a radial length of about 0.060 inches for a 20,000 RPM, 2-pole pair motor calculates to a loss of about 80 watts which for a 50 watt output constitutes a 61.5% loss of the input power. It should be recognized that the loss calculated above is for eddy current losses and has assumed that no magnetic saturation occurs resulting in other losses. The loss due to eddy currents is significant in itself and must be considered in the design of the motor.
Thus, in electrical motors which utilize varying magnetic fields, the magnetic flux return path would desirably be designed to carry any expected magnetic fields in the device with lower loss. The expected magnetic fields therefore dictate the requirements of the magnetic flux return path as to the material permeability and geometry necessary to channel a fluctuating magnetic field with low losses.